A storage device, such as a disc drive, typically uses a servo system to position a read or write head over a recording track were digital information is stored. The servo system relies on servo information stored either on a dedicated disc surface in a multidisc system or in servo sectors that are radialy dispersed throughout each disc. The servo information gives coarse position information such as the track number of the track that the head is positioned over and possibly the angular sector that the head is positioned over. The servo information also includes fine position information that describes the radial location of the head within a track.
The fine position information is generally stored using a servo field pattern that is a combination of several servo fields. There are several types of servo field patterns, including "null-type" servo patterns, "split-burst amplitude" servo patterns, and "phase type " servo patterns.
A null-type servo pattern includes at least two fields, which are written at a known phase relation to one another. The first field is a "Phase" or "Sync" field, which is used to lock the phase and frequency of the read channel to the phase and frequency of the read signal. The second field is a position error field, which is used to identify the distance of the head from the track centerline.
For a null-type position error field, the magnetization pattern is such that when the head is directly straddling a track centerline, the amplitude of the read signal is ideally zero. As the head moves away from the desired track centerline, the amplitude of the read signal increases. When the head is half way between the desired track centerline and the centerline of an adjacent track, the read signal has a maximum amplitude.
The magnetization pattern on one side of a track centerline in the null-type position error field is written 180 degrees out of phase to the magnetization pattern on the other side of the track centerline. Thus, the phase of the read signal in the position error field relative to the phase of the read signal from the sync field indicates the direction that the head is displaced from the track centerline.
To control the servo system, a single position error value is determined in each servo sector. Such position error values are typically created by demodulating the read signal associated with the position error field. Typically, the magnitude of a position error value indicates the distance of the head from the track centerline, and the sign of the position error value indicates the direction of the head's displacement.
Demodulation of the read signal from a null-type pattern has, in the past, always been a synchronous process. In a synchronous process, the exact phase of the read signal of the position error field is known from the phase field read signal because the phase field is written at a known and fixed phase relation to the position error field. A phase locked loop (PLL) is typically used to acquire the phase of the phase field, and this phase information is used for demodulating the position error field signal. The phase field must therefore be sufficiently long to enable the PLL to lock on to the phase and frequency of the read signal. For example, the phase field may be 3 to 4 times longer than the position error field.
Ensuring a consistent phase relationship between the phase field and the position error field in each servo sector of a disc is critical to accurate positioning of the head. If the phase between these two fields is not consistent at each servo field, a different position error value will be obtained at two different servo fields even though the head remains in the same radial position within the track. To insure this consistency, great effort and expense has been expended toward building consistent phase locked loops that operate the same in every servo field.
The position error value generated by the servo system ideally changes linearly as the head moves radially across the track. Such linear changes simplify the calculations needed to determine the amount by which the head must be moved to bring it to a desired location. In general, servo systems do not produce position error values that change linearly as the head moves radially across a track. In particular, as a head moves radially across a track, the read signal generated by the read head tends to fluctuate due to the geometry of the head. To reduce the effects of such fluctuations, the prior art has used automatic gain control systems to automatically adjust the gain of the servo loop so that it remains constant at all track positions for the head. The amount of gain that must be introduced by the automatic gain control is set by a control circuit that must be initialized either at disc burn-in or periodically during the life of the disc drive.
Thus, in the prior art, a great deal of effort has been expended on normalizing the position error value so that it is consistent across different servo areas. This has resulted in complex and costly structures that drive up the cost of the disc drive.